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United States Patent |
6,078,046
|
Mori
,   et al.
|
June 20, 2000
|
Apparatus for measuring electron beam intensity and electron microscope
comprising the same
Abstract
In an apparatus for measuring an intensity of an electron beam, the
electron beam is made incident a fluorescent screen and a part of the
incident electron beam is converted into an optical image, and the optical
image formed on the fluorescent screen is picked-up by a television camera
by a reflection mirror having a central hole and being inclined with
respect to a direction into which the optical image is made incident upon
the reflection mirror. A part of the electron beam transmitted through the
fluorescent screen and passing through the central hole of the reflection
mirror is made incident upon a Faraday case, which generates an electric
current representing an intensity of the electron beam impinging upon the
fluorescent screen. In the transmission type electron microscope, after
measuring an intensity of the electron beam impinging upon the fluorescent
screen, after removing the fluorescent screen, reflection mirror and
Faraday cage from an optical axis, a shutter is opened to expose a
photographic film to the electron beam for a time period which provides an
optimum exposure on the basis of the measured intensity of the electron
beam.
Inventors:
|
Mori; Hirotaro (Suita, JP);
Yoshida; Kiyokazu (Toyonaka, JP)
|
Assignee:
|
Osaka University (Suita, JP)
|
Appl. No.:
|
081778 |
Filed:
|
May 20, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
250/311; 250/397 |
Intern'l Class: |
G01K 001/08 |
Field of Search: |
250/311,397
|
References Cited
U.S. Patent Documents
5517033 | May., 1996 | Krivanek et al. | 250/311.
|
5811805 | Sep., 1998 | Osakabe et al. | 250/311.
|
Primary Examiner: Nguyen; Kiet
Attorney, Agent or Firm: Venable
Claims
What is claimed is:
1. An apparatus for measuring an intensity of an electron beam comprising:
a fluorescent screen for receiving an electron beam whose intensity is to
be measured and converting a part of the received electron beam into an
optical image;
a reflection mirror for reflecting the optical image produced by the
fluorescent screen into a direction which is inclined with respect to a
direction into which the optical image is made incident upon the
reflection mirror;
an image observing means for observing the optical image reflected by said
reflection mirror; and
an electron detecting means for receiving an electron beam transmitted
through said fluorescent screen and reflection mirror and converting the
received electron beam into an electric signal which is a measure for an
intensity of the electron beam impinging upon the fluorescent screen.
2. An apparatus according to claim 1, wherein said reflection mirror has a
hole at a center thereof, and an electron beam passing through the hole is
detected by the electron detecting means.
3. An apparatus according to claim 1, wherein said electron detecting means
includes a Faraday cage generating an electric current which represents an
intensity of the incident electron beam.
4. An apparatus according to claim 3, wherein said electron detecting means
further includes a digital picoammeter which receives the electric current
generated by said Faraday cage.
5. An apparatus according to claim 2, wherein said electron detecting means
includes a Faraday cage generating an electric current which represents an
intensity of the incident electron beam.
6. A transmission type electron microscope comprising:
an electron gun generating an electron beam;
a condenser lens system for converging the electron beam generated by the
electron gun and projecting the converged electron beam onto a specimen;
an objective lens system for receiving the electron beam transmitted
through the specimen and forming an electron beam image of the specimen;
an enlarging lens system for enlarging the electron beam image formed by
said objective lens system;
a fluorescent screen for receiving the electron beam image projected by
said enlarging lens system and converting the received electron beam image
into an optical image;
a reflection mirror for reflecting the optical image generated by said
fluorescent screen into a direction which is inclined with respect to a
direction into which said optical image is made incident upon the
reflection mirror;
an image observing means for receiving the optical image reflected by said
reflection mirror and producing an image of the specimen which can be
observed by an operator;
an electron detecting means for receiving an electron beam transmitted
through said fluorescent screen and reflection mirror and converting the
received electron beam into an electric signal which is a measure for an
intensity of the electron beam transmitted through the specimen and
impinging upon the fluorescent screen;
an image record medium arranged to receive and record the enlarged electron
beam image projected by said enlarging lens system; and
a shutter arranged removably between said enlarging lens system and said
image record medium to expose selectively the image record medium to the
enlarged electron beam image;
whereby said fluorescent screen, reflection mirror and electron detecting
means are arranged removably between said enlarging lens system and said
shutter, and an exposure of the enlarged electron image to the image
record medium is controlled on the basis of said electric signal generated
from the electron detecting means.
7. An electron microscope according to claim 6, wherein said reflection
mirror has a hole at a center thereof, and an electron beam passing
through the hole is detected by the electron detecting means.
8. An electron microscope according to claim 6 or 7, wherein said electron
detecting means includes a Faraday cage generating an electric current
which represents an intensity of the incident electron beam.
9. An apparatus according to claim 8, wherein said electron detecting means
further includes a digital picoammeter which receives the electric current
generated by said Faraday cage.
10. An electron microscope according to claim 6, wherein said image
observing means comprises a television camera for picking-up the optical
image of the specimen formed on the fluorescent screen by means of said
reflection mirror to derive an image signal, and a monitor for receiving
the image signal to display an image of the specimen.
11. An electron microscope according to claim 7, wherein said electron
detecting means includes a Faraday cage generating an electric current
which represents an intensity of the incident electron beam.
12. An apparatus according to claim 5, wherein said electron detecting
means further includes a digital picoammeter which receives the electric
current generated by said Faraday cage.
13. An apparatus according to claim 11, wherein said electron detecting
means further includes a digital picoammeter which receives the electric
current generated by said Faraday cage.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a technique for measuring an
intensity of an electron beam, and more particularly relates to an
apparatus for measuring an intensity of an electron beam and to an
electron microscope including the apparatus for measuring an intensity of
an electron beam.
2. Related Art Statement
There has been proposed a transmission type electron microscope comprising
an electron gun producing an electron beam, a condenser lens system for
converging the electron beam and projecting the thus converged electron
beam onto a specimen, an objective lens system for forming an electron
beam image of the specimen, an enlarging lens system for forming an
enlarged electron beam image of the specimen and projecting the enlarged
electron beam image, a fluorescent screen converting the enlarged electron
beam image of the specimen into an optical image of the specimen, and a
shutter arranged between the enlarging lens system and a photographic
film.
In such a transmission type electron microscope, contract and brightness
vary over a very wide range, and therefore it is necessary to provide a
certain index for projecting an optimum electron beam image of the
specimen onto an image record medium, for example, a photographic film. In
a remote observing system in which an optical image of a specimen is
observed with a television camera, it is particularly difficult to judge
an optimum exposure accurately based on a brightness of the fluorescent
screen through experiences, because the brightness of the optical image
changes in accordance with a sensitivity of the television camera. It is
important for the transmission type electron microscope to provide a
precise control for the exposure by means of which an operator can have a
confidence that excellent photographs can be obtained. It has been
proposed to arrange temporally a detector in front of the optical image
monitoring fluorescent screen only when an intensity of the electron beam
is to be measured. It has been further proposed to measure an intensity of
an electron beam flowing into the fluorescent screen by means of a plate
electrode built-in the screen.
In the former method, an optical image of a specimen could not be monitored
by the television camera during the electron beam intensity measurement,
because the detector is arranged in front of the fluorescent screen. That
is to say, both the observation of the optical image and the measurement
of the electron beam intensity could not be performed simultaneously. The
latter method has been generally used in conventional electron microscopes
having low electron beam energy, but could not be applied to ultra-high
voltage electron microscopes using an electron beam having high energy
owing to a reason that a detected current is no more proportional to an
intensity of the electron beam. This is due to a fact that a difference
between electrons trapped by the fluorescent screen and secondary
electrons emitted from the florescent screen is detected. Therefore, in an
extreme case, a polarity of the detected current might be reversed, and
the detected signal could not be used any more.
In order to perform the measurement of the intensity of the electron beam
effectively for a diffraction image and an electron beam image having a
high contrast, it is preferable to apply the center preponderant
measurement system, almost equal to the spot measurement.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate these disadvantages
of the conventional methods, and provide a novel and useful apparatus for
measuring an intensity of an electron beam, while at the same time an
observation of an optical image can be performed without any operations.
It is another object of the invention to provide a novel and useful
electron microscope including the apparatus for measuring precisely an
intensity of an electron beam intensity and being capable of exposing an
image record medium optimally.
According to the invention, an apparatus for measuring an intensity of an
electron beam comprises:
a fluorescent screen for receiving an electron beam whose intensity is to
be measured and converting a part of the received electron beam into an
optical image;
a reflection mirror for reflecting the optical image produced by the
fluorescent screen into a direction which is inclined with respect to a
direction into which the optical image is made incident upon the
reflection mirror;
an image observing means for observing the optical image reflected by said
reflection mirror; and
an electron detecting means for receiving an electron beam transmitted
through said fluorescent screen and reflection mirror and converting the
received electron beam into an electric signal which is a measure for an
intensity of the electron beam impinging upon the fluorescent screen.
According to the invention, a transmission type electron microscope
comprises:
an electron gun generating an electron beam;
a condenser lens system for converging the electron beam generated by the
electron gun and projecting the converged electron beam onto a specimen;
an objective lens system for receiving the electron beam transmitted
through the specimen and forming an electron beam image of the specimen;
an enlarging lens system for enlarging the electron beam image formed by
said objective lens system;
a fluorescent screen for receiving the electron beam image projected by
said enlarging lens system and converting the received electron beam image
into an optical image;
a reflection mirror for reflecting the optical image generated by said
fluorescent screen into a direction which is inclined with respect to a
direction into which said optical image is made incident upon the
reflection mirror;
an image observing means for receiving the optical image reflected by said
reflection mirror and producing an image of the specimen which can be
observed by an operator;
an electron detecting means for receiving an electron beam transmitted
through said fluorescent screen and reflection mirror and converting the
received electron beam into an electric signal which is a measure for an
intensity of the electron beam transmitted through the specimen and
impinging upon the fluorescent screen;
an image record medium arranged to receive and record the enlarged electron
beam image projected by said enlarging lens system; and
a shutter arranged removably between said enlarging lens system and said
photographic film to expose selectively the image record medium to the
enlarged electron beam image;
whereby said fluorescent screen, reflection mirror and electron detecting
means are arranged removably between said enlarging lens system and said
shutter, and an exposure of the enlarged electron image to the image
record medium is controlled on the basis of said electric signal generated
from the electron detecting means.
In a preferable embodiment of the electron beam intensity measuring
apparatus and the electron microscope according to the invention, said
reflection mirror is formed by a flat mirror having a center hole and said
electron detecting means is formed by a Faraday cage.
Further, according to the invention, said image observing means preferably
comprises a television camera for picking-up the optical image of the
specimen reflected by said reflection mirror to derive an image signal and
a monitor for receiving the image signal to display an image of the
specimen.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an embodiment of the apparatus for
measuring an intensity of an electron beam according to the present
invention;
FIG. 2 is a schematic view illustrating an embodiment of the transmission
type electron microscope according to the invention; and
FIG. 3 is a schematic view depicting a major part of the microscope shown
in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a schematic view showing an embodiment of the apparatus for
measuring an intensity of an electron beam according to the invention. It
should be noted that this drawing is represented simply for the sake of
explanation, and therefore shapes and dimensions thereof are different
from those of an actual device.
An electron beam whose intensity is to be measured is made incident upon a
fluorescent screen 1, and a part of the incident electron beam is
converted into an optical image. The optical image thus formed on the
fluorescent screen 2 is picked-up by a television camera 3 by means a
reflection mirror 2 having a central hole 2a and an optical lens 4. The
reflection mirror 2 is arranged to be inclined with respect to a direction
in which the optical image is made incident upon the reflection mirror 2.
In the present embodiment, the reflection mirror 2 is inclined by 45
degrees with respect to the incident optical axis. In this manner, the
optical image formed on the fluorescent screen 2 by the incident electron
beam can be displayed on an image monitor 5. Since the reflection mirror 2
is arranged far from the focal plane of the optical lens 4, the central
hole 2a does not affect a quality of the image displayed on the image
monitor 5.
A part of the incident electron beam transmitted through the fluorescent
screen 1 and the center hole 2a of the reflection mirror 2 is made
incident upon an electron detecting device 6. In the present embodiment,
the electron detecting device 6 is formed by a Faraday cage. Then, the
Faraday cage 6 generates an electric current whose amplitude represents an
intensity of the electron beam impinging upon the Faraday cage. The
amplitude of this electric current is detected by a digital picoammeter 7
and an output of this digital picoammeter 7 is supplied to a control unit
8 including a display device. As stated above, the amplitude of the
current signal supplied by the Faraday cage 6 represents an intensity of
the electron beam impinging upon Faraday cage, an intensity of the
electron beam being incident upon the fluorescent screen can be measured
by suitably processing the signal supplied from the digital picoammeter 7.
FIG. 2 is a schematic view showing an embodiment of the electron microscope
according to the invention and FIG. 3 is a schematic view illustrating a
major portion of the electron microscope.
As shown in FIG. 2, the electron microscope 11 comprises a high voltage
tank 12, in which an electron gun 13 and accelerating tubes 14. The high
voltage tank 12 is supported by a frame member 15 by means of a damper 16.
Below the high voltage tank 12, is arranged a major section 17 including
condenser lens system, specimen chamber, objective lens system, enlarging
lens system, fluorescent screen, reflection mirror, shutter, film support
and so on.
FIG. 3 is a schematic view the major section 17 of the electron microscope
shown in FIG. 2. In FIG. 3, the assembly in the high voltage tank 12 is
represented by an electron gun 21. An electron beam emitted by the
electron gun 21 is focused by a focus lens 22 and is made incident upon a
specimen 23. The electron beam transmitted through the specimen 23 is
collected by an objective lens 24 to form an electron beam image. The
electron beam image thus formed is enlarged by an enlarging lens 25 and
the enlarged electron image is made incident upon a fluorescent screen 26.
A part of the electron beam impinging upon the fluorescent screen 26 is
converted into light and optical image is formed on the fluorescent screen
26, and most of the electron beam is transmitted through the fluorescent
screen 26.
The optical image formed on the fluorescent screen 26 is picked-up by a
television camera 28 by means of a reflection mirror 27 having a central
hole 27a. The reflection mirror 27 is inclined with respect to a direction
into which the optical image is made incident upon the reflection mirror.
In this manner, the optical image formed on the fluorescent screen 26 can
be monitored.
A part of the electron beam transmitted through the fluorescent screen 26
passes through the central hole 27a of the reflection mirror 27 and is
made incident upon a Faraday cage 29. As explained above, the Faraday cage
29 produces an electric current signal which represents an intensity of
the electron beam impinging upon the Faraday cage through the fluorescent
screen 26 and the central hole 27a of the reflection mirror 27. The thus
generated current signal is measured by a digital picoammeter (AM) 30 and
an output signal of the digital picoammeter is supplied to a control
computer (CPU) 31. Then, the control computer 31 generates a control
signal which represents an intensity of the electron beam impinging upon
the fluorescent screen 26.
Below the fluorescent screen 26, there is arranged a shutter 32 which can
prevent the electron beam from being transmitted therethrough and being
made incident upon an image record medium, e.g. a photographic film 33
supported by a film holder 34.
According to the invention, the fluorescent screen 26, reflection mirror 27
and Faraday cage 29 are arranged on a housing schematically shown by a
reference numeral 35. The housing 35 is arranged to be movable with
respect to an optical axis extending from the fluorescent screen 26 and
the photographic film 33. That is to say, the housing 35 can assume a
first position in which the fluorescent screen 26, reflection mirror 27
and Faraday cage 29 are inserted into the optical axis, and a second
position in which all the above elements 26, 27 and 29 are removed from
the optical axis and thus the electron beam may be directly made incident
upon the photographic film 33.
In the present embodiment, the fluorescent screen 26 is formed by applying
P22 fluorescent material powder on a sapphire substrate having a thickness
of 0.4 mm. A diameter of the central hole 27a of the reflection mirror 27
is 1 mm. An accelerating voltage is set to 3000 KV.
In case of taking photographs of the electron beam image on the
photographic film, it is necessary to adjust and control an exposure
amount for the photographic film in a precise manner. To this end,
according to the invention, the housing 35 is set into the first position
and an intensity of the electron beam impinging upon the fluorescent
screen 26 is measured. During this measurement, the operator can always
monitor the optical image formed on the fluorescent screen 26.
Next, upon taking a photograph, the housing 35 is removed into the second
position such that the electron beam emanating from the enlarging lens
system 25 can be made incident upon the photographic film 33 for a time
interval during which the shutter 32 is driven to remove from the optical
axis under a control of the control computer 31. As stated above, since an
intensity of the electron beam has been measured, the control computer 31
can control the shutter 32 in such a manner that an optimal exposure can
be effected. It should be noted that an exposure for the photographic film
33 may be controlled by adjusting an intensity of the electron beam
emitted by the electron gun 21. Furthermore, an exposure for the
photographic film 33 may be adjusted also by controlling a magnetic
excitation of the condenser lens system 22.
The present invention is not to the above embodiments and many alternations
and modifications may conceived by those skilled in the art within the
scope of the invention. For example, the electron beam intensity measuring
apparatus according to the invention may be used in other devices than the
electron microscope such as an electron beam lithography apparatus, in
which both the observation of the optical image and the measurement of an
intensity of an electron beam have to be conducted simultaneously.
In the above embodiments, the hole is formed at a center of the reflection
mirror, but a thickness of a center portion of the reflection mirror may
be deceased such that an electron beam can practically pass the thin
portion of the reflection mirror.
Moreover in the above embodiments, the Faraday cage is used as the electron
detecting device, but any other electron detecting device may be also
used.
In the embodiments explained above, the reflection mirror is arranged to
reflect the optical image into a direction which is inclined by 90 degrees
with respect to the incident direction, but the reflection mirror may be
arranged to reflect the optical image into a direction which is inclined
with respect to the incident direction by an angle other than 90 degrees.
Furthermore, an observing window may be formed in a housing of the
electron microscope so that the optical image can be observed by naked
eyes.
In the apparatus for measuring an intensity of an electron beam according
to the present invention, both the observation of the optical image and
the measurement of the electron beam intensity can be performed
simultaneously without requiring any operation of a user. Moreover, in the
transmission type electron microscope according to the invention, the
optimum exposure can be accurately obtained upon the electron beam image
recording the image record medium. When the Faraday cage is arranged such
that an electron beam positioned at a center of a field of view is made
detected, the center preponderant measurement almost equal to the spot
measurement can be performed.
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